Electrostatically charged fluidized bed apparatus

ABSTRACT

Electrostatically charged fluidized bed apparatus having a porous membrane through which a continuous stream of air flows upwardly to fluidize powder in a bed above the membrane. A conductive web consisting of a layer of conductive particles incorporated into the membrane is maintained at a potential of at least 5,000 volts so that the stream of air carries electrical charges to powder suspended in the fluidized bed. Alternatively, the conductive web may be a fabric of conductive fibers mounted beneath and close to the membrane.

I United States Patent 1151 3,670,699

Sargent 1 June 20, 1972 [54] ELECTROSTATICALLY CHARGED 3,396,699 8/1968Beebe et al. ..117/17 FLUIDIZED BED APPARATUS 3,537,426 11 1970 Spilleret al ..1 18/629 3,496, 11 21970 Ch 1 ..1186 [72] Inventor: Jerald P.Sargent, Lake Elmo, Minn. 9 me ar 2] [73] Assignee: Minnesota Mining andManufacturing Primary Exam ner-Mer in Stein Company, St. Paul, Minn.Assistant ExaminerLeo Millstein [22] Filed: June 24 1970Attorney-Kinney, Alexander, Sell, Steldt & Delahunt 211 Appl. No.:49,241 57 ABSTRACT Electrostatically charged fluidized bed apparatushaving a [52] U.S. Cl. ..118/629, 1 l7/l7, l l8/DlG. 5, poro membranethrough which a continuous stream of air H8/627 flows upwardly tofluidize powder in a bed above the mem- [51] Int. Cl ..B05b 5/02 brarm Aconductive web consisting f a layer f conductive [58] Field of Search..1 18/628, 629, 639, 640, 400.5, particles incorporated into themembrane is maintained at a l 18/ 624; 1 17/17 potential of at least5,000 volts so that the stream of air carries electrical charges topowder suspended in the fluidized bed. [56] References CitedAlternatively, the conductive web may be a fabric of conduc- UNITEDSTATES PATENTS tive fibers mounted beneath and close to the membrane.

3,248,253 4/1966 Barford et al ..1 18/4005 6 Claims, 2 Drawing figuresPATENTEnJuH 20 me IIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIII T m .NME m T I/A 0M. mm M 3 m ELECTROSTATICALLY CHARGED FLUIDIZED BED APPARATUS FIELD OFTHE INVENTION This invention relates to electrostatically chargedfluidized beds for coating articles using powdered resins.

BACKGROUND TO THE INVENTION It is known that an electrostaticallycharged fluidized bed provides certain advantages over conventionalfluidized beds for coating articles with powdered resins. For example,the article to be coated need not be preheated, and powder can easily beselectively removed from the article before heating to fuse thedeposited powder. If there is a defect in the fused coating, powder isreadily applied to the defect and removed from areas already of adequatecoating thickness, after which the article is heated to fuse the newlydeposited powder. With a little practice and care, it is difiicult todetennine that the coating was ever defective.

Electrostatically charged fluidized bed devices are shown in U.S. Pat.Nos. 3,248,253 (Barford et al.), 3,336,903 (Point) and 3,339,699 (Beebeet al.). In these devices, the powder is electrostatically charged bymeans of metal electrodes located within the fluidized bed. A device ofthis general nature marketed by Electrostatic Corporation has a metalgrid suspended in the fluidized bed a short distance above the porousmembrane through which gas is forced upwardly to fluidize powder in thebed. When a grounded article is positioned above such a fluidized bed,powder is attracted to and deposited upon the article, but not indesirably uniform thickness.

Another shortcoming of electrostatically charged fluidized beds of theprior art is the high voltage required for efficient performance.Manufacturers of devices now on the market recommend the use. ofvoltages up to about 120,000 volts.

THE PRESENT INVENTION The present invention differs from devices of theprior art by the unique manner in which the powder of the fluidized bedis electrostatically charged and by the uniformity of resultantcoatings. Although the present invention still requires a high voltage,good efficiency is attained at voltages relatively lower than are neededin the prior art, e.g., only l0,000-20,000 volts, and the user is betterprotected from the high voltage.

These advantages are realized by incorporating into the porous membraneof an otherwise conventional fluidized bed apparatus a web of moderatelyconductive particles or fibers, which web may have a resistance of aboutl-l ,000,000 ohms and contains innumerable sites for charges whensubjected to a high voltage. The conductive web may be incorporated intothe porous membrane by coating one or both sides of the membrane with adispersion of graphite particles or fibers, using any coating technique,e.g., by spraying, brushing or dipping. Whatever coating technique isused, it is important that the conductive particles of the dried coatingbe contiguous over the entire coated area, but not at such a density asto appreciably interfere with the gas-transmissive nature of themembrane. Alternatively, the chargeable web may consist of a woven ornonwoven fabric of graphite fibers positioned horizontally beneath andclosely adjacent to the porous membrane in the path of the gas whichfluidizes the powder, again such that the upward flow of gas through theweb and porous membrane is not appreciably inhibited.

THE DRAWING FIG. 1 is a schematic central section of anelectrostatically charged fluidized bed apparatus embodying the presentinvention, and

FIG. 2 is a schematic central section of another embodi ment of thepresent invention employing a pair of fluidized beds.

The fluidized bed apparatus illustrated in FIG. I has four rectangularside walls 9, a bottom wall 10 and a porous membrane 11 to provide acontainer 12 and a chamber 13 into which a gas such as air iscontinuously supplied under pressure through a conduit 14. The gas flowsupwardly through the membrane 11 at a volume suflicient to suspendfree-flowing powder 15 as a fluidized bed within the container 12. Thewalls 9 and 10 are made of an insulating material such as athermoplastic resin. The membrane 11 rests on Supports 16 integrallyformed with the side walls 9, and a bead of thermosetting epoxy resin 17around the entire upper periphery of the membrane 11 holds the membranein place and seals it against substantial leakage at its peripheries.

The entire under face of the membrane 11 has been coated to provide acontinuous web 18 of conductive particles. One post of a high-voltageresistor 19 is electrically connected to the conductive web 18 by aconductive coil spring 20 which is under mild compression and held inplace by a globule of thermosetting epoxy resin to insure goodelectrical contact with the web. The other post has an electrical lead21 to a source of high voltage.

Another embodiment of this invention is shown in FIG. 2 employing a pairof identical fluidized beds which are of the same construction as theapparatus of FIG. 1 except as noted. Each of the pair has a porousmembrane 1 1a sealed between a low side wall 9a and a high side wall 9b.A thin adhesive tape having a metal foil backing 25 provides a smoothjoint between each membrane 110 and high side wall 9b. The entireexposed surface of each membrane 11a, metal foil 25 and high side wall9b is coated with conductive particles.

When a grounded article 26 is held in the position shown and theconductive webs 18a are charged to the same potential, powder 15a isattracted from both fluidized beds, thus coating all surfaces of theentire article 26 without changing its position. This makes theapparatus of FIG. 2 especially adapted to conveyor lines. In contrast,it is usually necessary EXAMPLE I A fluidized bed apparatus asillustrated in FIG. 1 has been constructed using polymethylmethacrylateof one-fourth inch thickness for the side walls 9 and base wall 10. Theporous membrane 11 was porous high-density polyethylene ofthreesixteenths inch thickness having a particle retention of 25 micronsand larger which is marketed as a fluidizing type membrane by PorousPlastics Limited, Essex, England under the tradename Vion." Thecontainer 12 measured 6 inches on a side and was 4.5 inches in height,and the chamber I3 was 2.5 inches in height.

The entire under surface of the membrane 11 was coated to provide amoderately conductive web 18 by wiping the surface with a cloth soakedin a dispersion of graphite particles in alcohol, specifically dag"dispersion No. 154 of Acheson Colloids Company, Port Huron, Michigan.The dried web 18 completely masked the white color of the membrane 11and exhibited a resistance of about 50,000 ohms at an electrodeseparation of 1 inch. None of the black color of the graphite particlespenetrated to the opposite surface of the web. The resistor 19 had aresistance of I60 megohms and was rated for 30 watts. The lead 21 wasconnected to a negative DC potential of 13,000 volts.

Used in this apparatus was a powder comprising solidbisphenol-epichlorohydrin-type epoxy resin, isophthalyl dihydrazide anda catalytic amount of dicyandiamide plus flow-control agents. Fifty-fivepercent of the powder would pass 325 mesh (44-micron openings). Startingwith an inch of powder, a flow of dry air of 0.9 cubic foot per minutefluidized the powder to a height of about 2 inches.

When a grounded copper bar at room temperature was placed one-half inchabove the surface of the fluidized bed for 6 seconds, it was uniformlycovered with powder which fused when the article was heated in an ovenat 200 C for 15 minutes to provide a tough cured electrically insulatingprotective coating having a uniform thickness of about. 6 mils (0.15mm).

Good results were obtained when the article being coated was a differentmetal or was moderately conductive or was a dielectric such as ceramicor plastic. In each case adequate grounding of simple articles such asthe copper bar was obtained through the body of the person holding thearticle above the electrostatically charged fluidized bed. Articlescontaining sharp inside corners or small openings are desirablyconnected to a grounded strap.

This apparatus has been operated with the conductive web at a potentialranging from 5,000 to l60,000 volts. Below ,000 volts, the rate ofpowder deposit is unduly slow for commercial use, and above 160,000volts, there is danger of arcing to the article being coated, althoughresistor 19 minimizes the chance of injury to the operator. A good rateof powder deposit is readily achieved at l0,00030,000 volts, and thereusually is no advantage to increasing the potential above 30,000 volts.

EXAMPLE 2 EXAMPLE 3 When the apparatus described in Example 1 wasmodified by reversing the porous membrane 1 1 so that the graphiteparticle web 18 was on the upper face, no difference was noted inperformance. Neither was any difference noted when a graphite particleweb was applied to both faces of the porous membrane, and both webs wereconnected to the 13,000-volt potential. Of course, care must be taken inapplying more than one coating of graphite particles that thegas-transmissivity of the web is not reduced to the point that thepowder can no longer be fluidized under moderate air pressure.

EXAMPLE 4 The apparatus described in Example 1 was modified bysubstituting an identical porous membrane for membrane 11 except thatinstead of being continuous, the conductive web was provided by agraphite coating applied in a pattern of a circle 1 inch in diameterplus three concentric rings, each one-half inch in width and spaced fromeach other and from the circle by one-half inch. With each member of thepattern electrically connected to the l3,000-volt potential, even betteruniformity of powder deposit was realized, as compared to the apparatusof Example 1, but at some expense in rate of coating buildup. Toincrease the coating rate to equal that of Example 1, it was necessaryonly to increase the DC potential to lead 19. However, since it isdesired to keep that potential as low as possible in the interest ofeconomy and the operator's peace of mind, it is normally preferred tocoat all or most of the face of the porous membrane with conductiveparticles, unless absolute uniformity of coating is required.

The graphite particles have been coated on the membrane in otherpatterns, including a small circle at the center of the membrane, asmall square at the center and a snowflake. In each case powder from thefluidized bed deposits on a grounded article at a somewhat lower rate,and no advantage is seen in using such patterns. Regardless of thepattern of the conductive web, its area should be at least 2 percent ofthe area of the membrane, and preferably at least 20 percent.

EXAMPLE 5 Apparatus identical to that described in Example 1 wasconstructed except that the porous membrane was uncoated and a seconduncoated porous membrane was added to provide means for mounting afabric of graphite fibers. The conductive web provided by thiscarbonaceous fabric was coextensive with the membranes, and separatedthe two membranes by its thickness, i.e., about one thirty-second inch.The fabric had a resistance of about 2-3 ohms measured at an electrodeseparation of one inch, and was connected to the resistor 19 by a leadwire extending through the lower membrane.

When the fabric was at a potential of 13,000 volts, the performance wasequivalent to that obtained with the apparatus described in Example l. Abed of the powder of Example 1 fluidized to a height of 2 inchesdeposited sufficient powder on a grounded copper bar held one-half inchabove the powder for 6 seconds to provide, after curing in an oven, auniform coating about 8 mils (0.20 mm) in thickness. Increasing theexposure time to 15 seconds provided a coating which when cured in anoven had a thickness of about 24 mils (0.61 mm).

When a similar unheated grounded bar was held 2.5 inches above thesurface of the bed for 15 seconds and then placed in an oven, a curedcoating was obtained having a uniform thickness of about 17 mils (0.43mm).

When additional powder was added to raise the height of the fluidizedbed to 4 inches, an unheated grounded bar placed one-half inch above thebed for 15 seconds received a coating which when cured had a uniformthickness of about 26 mils (0.66 mm).

EXAMPLE 6 The apparatus of Example 5 was modified by sealing an uncoatedporous polyethylene membrane to the four side walls 9 2 inches above theposition of membrane 1 1. The free-flowing powder was placed on theadded upper membrane, thereby spacing the conductive fabric from thefluidized bed by about 2 inches. With the fabric at 13,000 volts, thepowder deposited on grounded articles held above the fluidized bed at arate slightly less than 50 percent of that obtained with the apparatusdescribed in either Examples 1 or 4. This apparent loss of efficiencycan be compensated by increasing the applied voltage, not so desirablefrom a point of view of safety, as well as economy.

EXAMPLE 7 When the apparatus of Example 6 was modified by spacing theextra uncoated membrane only one-half inch above the carbonaceousfabric, the efficiency was only slightly reduced as compared to theapparatus of Example 5.

EXAMPLE 8 The apparatus of Example 6 was modified by replacing (a) thecarbonaceous fabric and the two membranes which held it with (b) thesingle membrane of Example 1 and its graphite particle web. The sameloss of a little over 50 percent in efficiency was observed, which losscould likewise be compensated by increasing the applied voltage.

EXAMPLE 9 Any coating of particles having a resistance of 1-1 ,000,000ohms measured at an electrode spacing of 1 inch should be useful, aslong as the flow of the fluidizing gas is not unduly restricted. Toillustrate, the apparatus of Example 1 was modified: instead of thecoating of graphite particles, the under surface of the porous web 11was coated with Ransprep No. 100," a transparent coating materialmarketed by Ransberg Electrostatic Corporation which dried to provide amoderately conductive coating having a light amber color and a thicknessof less than 1 mil (25 microns). The resistance of the dried coating was750,000 ohms measured at an electrode spacing of 1 inch.

This coating interfered slightly with the flow of air through the porousmembrane, requiring slightly increased pressure at the air supply toprovide a 2-inch fluidized bed. At the increased air pressure thecoating efficiency was essentially equal to that in Example 1.

EXAMPLE l The apparatus of Example 1 was modified by replacing themembrane 11 with an uncoated porous stainless steel plate of one-eighthinch thickness having l5-micron openings. The resistance of this platewas less than 0.1 ohm measured at an electrode separation of one inch.When 13,000 volts were applied to the stainless steel plate, thefluidized bed was charged so that articles could be coatedelectrostatically. However, the efficiency was very poor. From this andother experiments, it was determined that the resistance of theconductive web 18, or of the porous membrane if the membrane isconstructed of conductive material in lieu of providing a conductiveweb, should not be less than about 1 ohm.

To provide a porous membrane which is useful for the purposes of thisinvention without a separate conductive web, one should construct themembrane out of material of only moderate conductivity such asconductive polyethylene.

in the apparatus of each of the foregoing examples, best results wereattained at a fluidized bed depth of about 1-3 inches and with thearticle to be coated about k to 4 inches above the surface of thefluidized bed. Closer distances permit rapid buildup. However, articleshaving sharp inside corners or small openings should be held at greaterdistances to insure reasonably uniform powder buildup on both externaland internal surfaces. Surfaces as much as two feet from a fluidized bedoperating as described in Example 1 receive an appreciable deposit ofpowder, but it is desirable for commercial purposes to hold the articleso that all of its surfaces are much closer to the bedthan thatdistance.

When the side walls of the apparatus used in Example 1 were extended andsufficient powder was added so that the fluidized bed was inches inheight, it was necessary to hold the article within about one-fourthinch of the surface of the bed of powder to obtain reasonable efficiencyat 13,000 volts.

At a distance of one-half inch, the rate of powder buildup was reducedto a third. Greatly increased voltage was necessary to enable reasonablyefficient deposit of powder on areas of the article which were not veryclose to the surface of such a deep fluidized bed.

EXAMPLE 1 1 Apparatus as illustrated in FIG. 2 was constructed in thesame manner as in Example 1 except as indicated below. Each of the highside walls 9b was 12 inches in height and extended 7% inches above themembrane 1 la. The low side walls 9a extended 2% inches above themembrane 11a. Each bed was 9 inches wide between side walls 9a and 9band 43 inches in length. The entire upper surface of each membrane 11awas coated with the same dispersion of graphite particles used inExample 1, but the web of graphite particles did not extend up the highside walls 9b.

With the powder used in Example 1 fluidized to a height of 2 inches andboth of the conductive webs 18a at a positive potential of 13,000 volts,a grounded copper bus bar measuring 3 feet by 4 inches by V4 inch wasmoved horizontally past the fluidized beds in the position of article 26as shown in FIG. 2, with the large dimension horizontal and theintermediate dimension vertical. All surfaces of the bus bar received adesirably uniform deposit of powder which cured to provide a tough,uniform protective coating when the bus bar was heated in an oven.

Apparatus of the type described in this example has been modified asshown in HQ 2 to continue the conductive membrane 18a up the high sidewalls 9b. As a result, powder was deposited at a more rapid rate ongrounded articles in the position of article 26in FIG. 2. Since some ofthe powder falls through the space between the two fluidized beds, it isdesirable that there be means for collecting and reusing it.

I claim:

1. Fluidized bed coating apparatus having side walls of insulatingmaterial and a horizontal porous membrane sealed to the side walls toform a container and means for producing an upward stream of gas throughthe membrane for providing a fluidized bed of free-flowing powder in thecontainer, which apparatus is characterized by the improvementcomprising a gas-transmissive, moderately conductive web consisting of alayer of contiguous conductive particles incorporated into the porousmembrane or a fabric of conductive fibers mounted horizontally beneathand close to the porous membrane in the path of gas produced by themeans for producing a stream of gas, which web contains innumerablesites for charges when subjected to a high voltage, and means forapplying to the web a potential of at least 5,000 volts so that gasflowing upward through the charged web carries electrical charges topowder suspended in the fluidized bed.

2. Fluidized bed coating apparatus as defined in claim 1 wherein theconductive web consists of a layer of graphite particles applied to theunder face of the porous membrane.

3. Fluidized bed coating apparatus as defined in claim 1 wherein theconductive web consists of conductive particles incorporated into theporous membrane, which particles are fibers.

4. Fluidized bed coating apparatus as defined in claim 1 wherein theconductive web consists of a woven or nonwoven fabric of graphitefibers.

5. Fluidized bed coating apparatus as defined in claim 1 wherein theconductive web has a resistance of about 1l,000,000 ohms measured at anelectrode spacing of 1 inch.

6. A pair of closely spaced fluidized bed coating apparati as defined inclaim 1, each having a relatively low opposite side wall positionedadjacent the relatively low side wall of the other, such thatelectrostatically charged powder in both beds is attracted to a groundedarticle intermediate the two relatively low side walls to deposit powderon the article from both sides.

2. Fluidized bed coating apparatus as defined in claim 1 wherein theconductive web consists of a layer of graphite particles applied to theunder face of the porous membrane.
 3. Fluidized bed coating apparatus asdefined in claim 1 wherein the conductive web consists of conductiveparticles incorporated into the porous membrane, which particles arefibers.
 4. Fluidized bed coating apparatus as defined in claim 1 whereinthe conductive web consists of a woven or nonwoven fabric of graphitefibers.
 5. Fluidized bed coating apparatus as defined in claim 1 whereinthe conductive web has a resistance of about 1-1,000,000 ohms measuredat an electrode spacing of 1 inch.
 6. A pair of closely spaced fluidizedbed coating apparati as defined in claim 1, each having a relatively lowopposite side wall positioned adjacent the relatively low side wall ofthe other, such that electrostatically charged powder in both beds isattracted to a grounded article intermediate the two relatively low sidewalls to deposit powder on the article from both sides.